U.S. Patents, such as U.S. Pat. No. 3,755,261 to Norman M. VanGulick and U.S. Pat. No. 3,891,606 to Irving Charles Kogon teach isocyanate prepolymer can be cured by the use of a complex of 4,4'-methylene dianiline and selected alkali metal salts, preferably sodium, potassium and lithium, and to include sodium chloride. Said complex is used in conjunction with a compound containing a source of active hydrogen, preferably a polyfunctional compound such as polyfunctional alcohol, e.g. glycerol. Optionally the combination of the complex and the source of active hydrogen is dispersed in an inert vehicle such as an organic liquid e.g. di(2-ethylhexyl)phthalate or tetra ethylene glycol dioctanoate. Unfortunately these amine salt complexes do not give satisfactory cures at relatively low temperatures 25.degree. to 30.degree. degrees above about 100.degree. C. with the more economical polyether glycol prepolymers, for instance those of polypropylene ether glycol or polyol.
Although the prior art describes the plasticizer or vehicles for the MDI-salt complex in broad terms, it did not teach that certain vehicles were superior to others or in effect acted catalytically to speed the rate of cure and to lower the cure activation temperature.